The present invention relates to a dropper-type stabilized power circuit, and specifically to a configuration which prevents a reverse current which flows from the output side to the input side in a low-saturation-type stabilized power circuit which utilizes a PNP-type transistor.
FIG. 15 shows a basic structure of a low-saturation-type series regulator 51, one of dropper-type stabilized power circuits, which utilizes a PNP-type transistor as a power transistor.
An emitter of a power transistor 11 is connected to an input terminal IN of the series regulator 51. Further, a collector of the power transistor 11 is connected to an output OUT of the series regulator 51.
A collector of a driving transistor 12 made up of an NPN-type transistor is connected to a base of the power transistor 11. Further, an emitter of the driving transistor 12 is connected to a GND.
An output terminal of an error amplifier 13 is connected to a base of the driving transistor 12. Further, an inverting input terminal of the error amplifier 13 is connected to a junction of a voltage dividing resistance R1 and a voltage dividing resistance R2 which are provided in series between the output terminal OUT of the series regulator 51 and the GND.
A non-inverting input terminal of the error amplifier 13 is connected to a standard power circuit 14 which generates a reference voltage Vref1. Further, a power voltage Vcc of the error amplifier 13 and the standard power circuit 14 is taken from the input side of the series regulator 51. Further, a high-temperature-leak compensating resistance REB is provided between the emitter and the base of the power transistor 11.
In the foregoing structure, a feedback voltage corresponding to an output voltage Vo which exists from the junction of the voltage dividing resistances R1 and R2 to the series regulator 51 is inputted to the error amplifier 13. Further, the error amplifier 13 compares the feedback voltage with the reference voltage Vref1 of the standard power circuit 14 and outputs a voltage according to the gap between the feedback voltage and the reference voltage Vref1, so that the error amplifier 13 adjusts a collector current of the driving transistor 12, that is, a base current of the power transistor 11. This adjustment increases/decreases the collector current of the power transistor 11, so that the output voltage Vo is stabilized.
The high-temperature-leak compensating resistance REB increases the collector current of the power transistor 11 by increasing a leak current of the driving transistor 12 at high temperatures, so that a rise of the output voltage Vo is prevented.
However, in the conventional dropper-type stabilized power circuit such as the foregoing series regulator 51 which utilizes the high-temperature-leak compensating resistance REB, there is a case where a voltage higher than input voltage (Vin) is applied to the output (OUT) of the stabilized power circuit from outside due to a misconnection etc. In this case, a base current in a reverse direction flows from the output side via the collector of the power transistor (11), the base of the power transistor (11), and the high-temperature-leak compensating resistance REB to the input side. Thus, the power transistor (11) becomes ON in a reverse direction, so that there exists a problem that the reverse current flows from the output side to the input side.
Japanese Unexamined Patent Publication No. 36711/1993 (Tokukaihei 5-36711) (published date: Feb. 12, 1993) discloses a following stabilized power circuit. In the stabilized power circuit, a diode is provided in parallel with a power transistor so that an output side is an anode and an input side is a cathode. Thus, the reverse current flows to the diode, so that the power transistor is protected.
However, for example, in a portable device which utilizes a battery, there is a case where it is possible to obtain an output of the stabilized power circuit from a body via a connection terminal such as an option. In this case, there is a possibility that a voltage higher than an input voltage (Vin) can be applied to the output (OUT) of the stabilized power circuit due to a misconnection etc. In such a case, since input of the stabilized power circuit is supplied by a battery, the battery is charged by the reverse current. Further, there is a possibility that the battery may ignite due to overcharge, depending on cases. Thus, even when the series regulator 51 which is arranged to have the high-temperature-leak compensating resistance REB has a diode for a by-puss like the foregoing publication, this problem cannot be solved.
The object of the present invention is to provide a dropper-type stabilized power circuit which can prevent a reverse current which flows from an output side to an input side, even when a voltage of an output side is higher than voltage of an input side with a high-temperature-leak compensating resistance provided.
The dropper-type stabilized power circuit of the present invention, in order to achieve the foregoing object, includes a PNP-type transistor; a high-temperature-leak compensating resistance provided between an emitter and a base of the power transistor; a compensating resistance switch provided in series with the high-temperature-leak compensating resistance between the emitter and the base; an output terminal which outputs a voltage; an input terminal to which a voltage is inputted; and compensating resistance switch controlling means for detecting a voltage of the output terminal and conducting the compensating resistance switch under normal operating conditions in which an input voltage of the input terminal is dropped so as to obtain an output voltage of the output terminal, and not conducting the compensating resistance switch when the detected output voltage is higher than the output voltage under normal operating conditions and is not less than a predetermined value which is set to be not more than value at which a current begins to flow from a collector to the base of the power transistor.
According to the invention, under normal operating conditions in which the input voltage is dropped so as to obtain the output voltage, the compensating resistance switch controlling means conducts a compensating resistance switch provided in series with the high-temperature-leak compensating resistance between the emitter and the base of the power transistor, and makes the high-temperature-leak compensating resistance ready to function. On the other hand, when a voltage of the output terminal becomes higher than the output voltage under normal operating conditions due to miss-connection etc. and becomes higher than the predetermined value which was set to be not more than a value at which a current begins to flow from the collector to the base of the power transistor, the compensating resistance switch controlling means does not conduct the compensating resistance switch so that a current does not flow to the high-temperature-leak compensating resistance.
Thus, even when a voltage of the output terminal is not less than the value at which a current begins to flow from the collector to the emitter of the power transistor, there is no current which flows from the collector via the base of the power transistor to the high-temperature-leak compensating resistance. Further, when the output voltage becomes high in a dropper-type stabilized power circuit, a base current of the power transistor is restrained generally, so that a base current of the power transistor is controlled so as not to flow under abnormal operating conditions in which a voltage of the output terminal is higher than a voltage of the input terminal. Thus, under abnormal operating conditions, the current which flows from the collector via the base of the power transistor to the paths other than the high-temperature-leak compensating resistance is restrained. By this, it is possible to prevent the power transistor from being ON in a reverse direction.
As a result, it is possible to provide the dropper-type stabilized power circuit which can prevent the reverse current which flows from the output side to the input side, when voltage of the output side is higher than voltage of the input side with the high-temperature-leak compensating resistance provided, For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.